The present invention relates to a scanning optical system to be employed in a laser beam printer or the like, and more particularly, to a reflection type scanning optical system in which a curved mirror is disposed instead of an f.theta. lens between a deflector and an imaging surface (i.e., a surface to be scanned).
An example of a reflection type scanning optical system is disclosed in Japanese Patent (JP) Provisional Publication No. SHO 62-253116. In the scanning optical system disclosed in the JP publication, a beam emitted from a light source is deflected by a polygonal mirror on a plane including the incident beam. The deflected beam is reflected by a curved mirror in a direction crossing a plane including the deflected beam to proceed towards a photoconductive drum. The curved mirror is formed such that an incident angle of the deflected beam gradually increases from a center toward a periphery so that the beam draws a linear locus on the photoconductive drum.
In order to provide a single curved mirror instead of an f.theta. lens which is generally employed in a scanning optical system, the curved mirror should have at least a function of correcting distortion and a function of correcting curvature of field in an auxiliary scanning direction, and also in a main scanning direction, to realize f.theta. characteristics. Further, when a polygonal mirror is employed, it is required that the curved mirror has a function of correcting a surface tilting error (the error caused by the tilting of the reflection surfaces of the polygonal mirror in the auxiliary scanning direction) of the polygonal mirror. In summary, the curved mirror is required to have a power in the auxiliary scanning direction in order to correct at least the curvature of field in the auxiliary scanning direction and the surface tilting error with remaining an excellent optical performance.
In the scanning optical system disclosed in the above JP publication, however, the degree of freedom in designing an optical system is relatively low since the curved mirror does not have functions of correcting the curvature of field in the auxiliary scanning direction or the surface tilting error of the polygonal mirror.